Standard duplex 15 A 125V electrical receptacle outlet devices contain identical sockets or receptacles each having slots configured to receive the blades (also commonly referred to as prongs or pins) of a plug device connected to an electrical appliance or other electrical load, whereby when the blades of the plug device are inserted in the corresponding slots, contact members located along the inner surface of the slots are in an electrical engagement with an outer surface of the blades. Conventionally, current is supplied to a receptacle through electrical wiring connecting between an electrical panel and the outlet, with the phase or “hot” wire or conductor connecting to a “live” terminal by a side terminal screw, push-in, screw-and-clamp, or other connecting system and bringing power to the outlet, while the return or neutral wire or conductor is connected to the neutral terminal via a similar connecting system in order to carry current from the outlet back to the electrical panel. Older NEMA 1-15R (National Electrical Manufacturers Association) non-grounding receptacle devices have positive and neutral contact slots, but lack a ground pin receiving slot, while newer NEMA 5-15R receptacle devices include a third grounding conductor receiving slot. Grounding receptacles are mandatory in most new construction and connect to a separate grounding terminal, and serve as an important additional safety device for the outlet.
In conventional socket-outlets or receptacles, the “hot” wire carrying alternating current (AC) to the outlet is connected so a current is available to the outlet as soon as the hot and neutral prongs of a power cord plug attached to an appliance are inserted in the appropriate slots of a socket or receptacle far enough for the outer surface of the prongs to make and establish an electrical contact with the contact members in the slots. While this arrangement is desirable because as a result power is almost instantaneously available to an appliance when the plug is inserted, a drawback is that the prongs only have to be inserted part-way into the outlet slots before an electrical contact with the forward end of the metal contact members located within the slots is established. The potential exists therefore for a gap to form between the front surface of the outlet and a partially dislodged plug body, in which the exposed plug prongs are live. The plug may slowly loosen over time, or an object might fall on and partially dislodge the plug, both of which could create an electrical hazard. In addition, the finger of a person attempting to plug in a lamp or other load item could accidentally slip between the front face of the outlet and the plug prongs when extended only partway into the outlet causing the person to be shocked and possibly resulting in serious injury or even death.
There is also the well-known danger of a curious young child or other person unwittingly inserting a knife blade or other object directly in the “live” receptacle slot. Since 2013, tamper resistant receptacles (TRRs′) having spring-loaded receptacle cover plates which close off the slots in the sockets until an equal pressure such as would be provided by a two-prong plug is applied to both hot and neutral slots at the same time have been required in new residential construction. While TRR's prevent the insertion of small objects in the individual slots particularly by small children, if two objects are inserted simultaneously, this safety feature can be overcome.
In addition to TRR's, many other attempts to reduce the risk of electrical shock from outlets and receptacles have been made, such as providing outlet covers over the outlet openings, or arrangements for more securely holding a cord prong in the outlet. Some outlets used outside of the U.S. have an on/off switch on the front of the outlet that must be manually activated. These devices do not automatically deactivate the outlet when a plug is removed, however, and are likely to be left on, and furthermore can be easily activated by a child. Additional safety measures that will ensure current is prevented from flowing to an outlet when not in use or when the plug becomes even partially dislodged therefore are desirable.
A related hazard is that in order for NEMA 5-15P grounding plugs having two current-carrying terminals or poles and a grounding pin to be connected to older NEMA 1-15R non-grounding receptacle outlet devices having two prong receiving slots but lacking a ground pin receiving slot, electrical plug adapters having a three-pin plug-receiving socket portion and a two-prong outlet connecting portion are commonly used. Such adapter devices also typically include a metal grounding tab which is intended to be connected to an electrical ground, such as a grounded cover or faceplate screw. Such adapter devices also typically include a metal grounding tab which is intended to be connected to an electrical ground, such as a grounded cover or faceplate screw.
Three-prong power cord plugs are mandated on major home appliances such as refrigerators, washing machines, microwave ovens, as well as on power strips and various other devices. Small appliances, however, such as electric shavers, lighting, blenders, crock pots, coffee makers, printers, and other generally portable or semi-portable consumer electronics and appliances, typically only require non-grounded two-prong plugs. Many such plugs are polarized such that the neutral prong is wider than the hot prong and therefore the plug can only be connected in a wall outlet in one orientation, while others that do not distinguish between neutral and line are unpolarized.
As a general rule, Class 1 appliances and devices having a metal-encased power supply require a three-prong power cord plug which ground plug is connected directly to the casing in order to protect users of such appliances from possible electric shock. Thus, if a hot wire in the metal case becomes loose and contacts the case, instead of the case becoming hot, if properly grounded the hot wire electricity is transferred to ground, tripping the breaker box and likely damaging the device but protecting the user from shock. Class II appliances are typically smaller appliances, and either are made of plastic, or include other safety features such as being “double insulated” so there is an extra layer of insulation between the live wire and outside casing in order to mitigate the shock risk. No single fault in a Class II appliance can result in dangerous voltages being exposed such that users can receive an electric shock while handling the device. While Class II appliances may be equipped with a three-prong plug, due to the lower cost and compact size of the two-prong plugs they are usually not provided. Thus, the added protection of a grounding plug is not afforded, and the risk of the plug becoming partially loosened from an outlet as described above remains.